1. Field of the Invention
The present invention relates to a trimming process for trimming a thin film resistor, for example, by a laser beam.
2. Description of Related Art
In recent semiconductor integrated circuits and hybrid integrated circuits, a functional laser trimming process has been used because the product processed by it has high precision output characteristics.
The trimming process by a laser beam allows an operator to trim or delineate an integrated circuit on a chip without any electrical contact with the integrated circuit. A major factor to determine the output characteristics of the integrated circuit is, for example, a resistance of a resistor of an integrated circuit. The desired resistance is obtained by the following steps: a proper resistance is initially set up, the integrated circuit is operated, and a resistance of the resistor is finely adjusted using cutting or trimming it by a laser beam till a desired resistance is obtained, while observing the output characteristics. This process is called functional laser trimming.
The process for adjusting a resistance of a resistor comes in two varieties. In a first resistance adjusting process, the resistance is adjusted by changing the direction of the lines of electric force. To this end, a resistive film is groove worked. In a second resistance adjusting process, a resistor network consisting of a plurality of resistor elements is used. To adjust a resistance of the resistor network, the resistor network is partially cut off to change a connection in the network.
A trimming mechanism of the trimming process based on a laser beam will be described for explaining what problem exists in a conventional laser beam trimming process. A conceptual illustration of a power distribution of an irradiating laser beam is given in FIG. 1A. In the figure, the abscissa of the graph represents an extension of a beam spot in the radial direction. The ordinate represents a power of the laser beam spot. During the trimming process, the beam spot is moved in the direction orthogonal to a plane of the drawing. As shown, a distribution of the laser power is a Gaussian distribution in which a peak power exists at point A as the center of the beam spot, and the peak power gradually decreases toward the peripheral side of the beam spot. FIG. 1B shows a state of a trimmed workpiece or material 1 when it is irradiated with a laser beam. Under this condition, trimmed material 1 absorbs energy of the laser beam. The absorbed laser energy is converted into a corresponding thermal energy, so that trimmed material 1 is heated. Portion 1a distanced from the center of the laser beam spot is a solidus region which is heated by the thermal energy resulting from the absorbed laser energy. An amount of heat value increases toward the center of the laser beam, and a melting point of the trimmed material is reached to form melting region 1b. In a region 1c in the vicinity of the center of the beam spot the temperature further rises above the melting point to reach a boiling point, and the trimmed material in this region is vaporized, so that the trimmed material in vaporizing region 1c is evaporated and cut off. In this way, the trimming process is performed. A vapor force during the vaporizing of the trimmed material pulls up part of melting region 1b, so that protruded portion 1d appears at the fringe of trimmed portion 2 of the trimmed material after the trimming process is completed, as shown in FIG. 1c. When a protecting film is applied over the substrate including the trimmed portion after the trimming process is completed, the protecting film loses the uniformity in thickness or becomes thin at protruded portion 1d. Sometimes, minute holes like pinholes or discontinuous or uncovering portions are found in the protecting film thereat.
The products manufactured by the conventional trimming process inevitably suffer from pinholes or uncovered portions of the protecting film and in this respect, are poor in humidity-resistance and hence will develop rust and other corrosion problems.